The present invention relates to a surface acoustic wave filter for use in a high-frequency circuit of a mobile communication device typified, for example, by a portable telephone or car telephone and, more particularly, to an easy-to-manufacture surface acoustic wave filter that has an excellent attenuation characteristic in the neighborhood of its pass band.
A resonator type surface acoustic wave (hereinafter referred to as SAW) filter is provided with reflectors on both sides of each interdigital transducer (IDT) to reflect surface acoustic waves, thereby generating standing waves.
With the provision of such reflectors, sawtooth or spike-like spurious pulses develop in the vicinity of the pass bandxe2x80x94this is attributable to the reflectors. The sizes and shapes of these spurious pulses depend on the configuration of the SAW filter, such as the number of IDTs used and the IDT spacing.
Further, in recent portable telephone or similar mobile communication systems the necessity for effective use of available frequency has lessened the space between transmitting and receiving frequency bands, that is, what is called a transition bandwidth, and there is the growing demand for SAW filters which provide increased transmission attenuation in the neighborhood of the pass band in such mobile communication systems.
FIG. 20 depicts a transversely-coupled, dual mode SAW filter disclosed in Japanese Patent Application Laid-Open Gazette No. 128519/91. This SAW filter has two resonators 201 and 202 disposed on a piezoelectric substrate (made principally of crystal) 200 across the direction of propagation of surface acoustic waves. The resonators 201 and 202 have reflectors 206 to 209 on both sides of interdigital transducers (IDTs) 203 and 204, and are excited in fundamental and second-order modes. Each reflector is graded or weighted by a different number of electrodes arranged in the direction of propagation of surface acoustic waves, thereby suppressing high-order modes and hence providing an improved spurious characteristic.
Incidentally, since the electrodes of the reflectors 203 to 209 in FIG. 20 are weighted along a sine curve, the development of filters encounters much difficulty, for example, in the design of electrode patterns, including simulations and in the creation of data on electrode patterns for use in the fabrication of photomasks.
It is therefore an object of the present invention to provide a SAW filter that permits suppression or elimination of the aforementioned spurious pulses to improve attenuation in the vicinity of the pass band and has an easy-to-develop and fabricate structure.
According to the present invention, there is provided a longitudinal-coupled SAW filter that has at least two interdigital transducers formed over a piezoelectric substrate and reflectors disposed on both sides of the interdigital transducers and in which the reflectors each comprise: a group of electrodes graded so that their number linearly varies with their position in the direction perpendicular to the direction of propagation of surface acoustic waves; and bus bars shorting the electrodes.
Since the electrode groups of the reflectors are graded so that the number of electrodes of each group varies with their position in the direction perpendicular to the direction of propagation of surface acoustic waves, the present invention allows ease in the design and development of the SAW filter, and permits reduction of the spurious pulses and hence provide improved attenuation in the vicinity of the pass band.